This ELU question motivated the following, but I abbreviate all clauses. Also, lest 'then' be written twice in the same sequence of events, interpret the adverb 'then' to precede the adverb 'thence'.
Unless is a kind of negative of if—think of [...] unless as very much like “if . . . not.” [...]
[1.] Unless, if A, [then] B, [thence] C.
[2.] In other words, if it is the case that if A, then B, then[ce] C;
otherwise, C. There are two conditionals operating here.
In general, unless X, Y = X, if not Y. = [3.] If not
Y, then X.
In [1.] above, X = if A, [then] B. And
Y = C.
Then using [3.], [1.] becomes:
[4.] If not
C, then if A, [then] B.
But [4.] differs from [2.]; so which is wrong?
Please explain and show all steps and thought processes.